Patent application Ser. No. 08/260,910 filed Jun. 15, 1994 (now U.S. Pat. No. 5,439,532) described air and fuel mixed and burned inside an emitter tube above an infrared (IR) emitter zone. The hot combustion gases then flow downward inside the emitter tube through the IR emitter zone to the emitter tube open bottom end and thence upward again through the IR emitter zone outside the emitter tube heating the emitter uniformly through a double pass. Uniform emitter temperature from the top to the bottom of the IR emitter zone is obtained because the higher heat loss at the upper end in the down pass is counter balanced by lower heat loss at the upper end in the upward pass. Fine tuning of the temperature profile can be obtained by using fins inside and/or outside of the emitter tube and adjusting the fin area as a function of length along the emitter zone.
In the earlier application, the fuel was injected from below in a central tube passing upward through the IR emitter zone. That is acceptable when using natural gas as the fuel. However when hydrocarbon fuels with larger hydrocarbon molecules are used (e.g. propane, butane, kerosene, or gasoline), pyrolysis in the hot zone can occur, resulting in carbon deposits inside the fuel tube. This problem can be avoided by injecting the fuel from above with a fuel tube running down through the center of the counterflow heat exchanger. The counterflow heat exchanger is not shown in FIG. 2 but is as shown in the earlier patent application.
The addition of a mixing nozzle enhances the fuel and air mixing in the combustion zone.